Differential effects of Cytochrome c and secondary metabolites on hexavalent chromium reduction in estuarine wetland sediments.

Cytochrome c (Cyt-c) is widely found in estuarine wetland sediments as an electron carrier. However, the mechanisms by which it influences hexavalent chromium (Cr6+) reduction, either independently or in conjunction with plant-derived secondary metabolites, have yet to be fully elucidated. The results revealed that Cyt-c addition significantly inhibited Cr6+ elimination, lowering abiotic and biotic removal rates by up to 2.1% (p < 0.05) and 1.8% (p < 0.05), respectively. Cyt-c drastically suppressed abiotic/biotic sediment reductant consumption, with AVS consumption reduced by 54.32%/51.32% and Fe2+ consumption reduced by 94.12%/89.99%, through electrostatic repulsion of Cr6+ oxyanions and preferential Fe2+ complexation. Metabolite amendment mitigated such inhibition, and SA outperformed PA due to its high humic acid proportion (45.5%) and four-fold higher antioxidant capacity. A newly developed three-pathway kinetic model revealed that Cyt-c selectively inhibits chemical reduction while facilitating microbial bioreduction under non-sterilized sediment conditions. These findings uncover a previously unrecognized inhibitory role of extracellular Cyt-c in Cr6+ attenuation and highlight the complex redox interplay among proteins, inorganic reductants, and plant metabolites. For bioremediation, this implies that native Cyt-c may constrain Cr6+ detoxification, whereas strategic use of high-performing species like SA could enhance remediation efficiency in contaminated coastal wetlands.